Experimental Demonstration of Energy-Chirp Compensation by a Tunable Dielectric-Based Structure

A tunable energy-chirp compensator was used to remove a correlated energy chirp from the 60-MeV beam at the Brookhaven National Laboratory Accelerator Test Facility. The compensator operates through the interaction of the wakefield of the electron bunch with itself and consists of a planar structure comprised of two alumina bars with copper-plated backs separated by an adjustable beam aperture. By changing the gap size, the correlated energy chirp of the electron bunch was completely removed. Calculations show that this device, properly scaled to account for the electron bunch charge and length, can be used to remove residual correlated energy spread at the end of the linacs used for free-electron lasers. The experimental results are shown to be in good agreement with numerical simulations. Application of this technique can significantly simplify linac design and improve free-electron lasers performance.

Figure 1

A planar tunable dechirper. (Left) The dashed red line in the center of the photo shows the electron beam trajectory. (Right) A schematic of the dechirper.

Figure 2

(a) Short-range wake potential calculated for four values of the dechirper gaps indicated on the plot. (b) Illustration of the definition of the dechirper strength using the linear fit of the wake potential between points $A$ and $B$ (the case with 1.9-mm dechirper gap was used). The black curve shows the profile of the electron charge distribution.

Figure 3

(a) Calculated short-range wake potential produced by the electron bunch with a $150\text{−}\mu \mathrm{m}$-flattop (black curve) in a dechirper with a 6.35-mm dielectric layer (red curve) and a dechirper with a $20\text{−}\mu \mathrm{m}$ dielectric layer (purple and green curves). The dechirper gap is indicated on the plot for each case. (b) Spectrum of the wake potentials. The colors of the spectral lines in this panel match the colors of the wakefield curves in panel (a).

Figure 4

Schematic of the experimental setup.

Figure 5

(a) Phosphor screen images of the beam passing through the dechirper with gap sizes of 5.8, 2.8, 1.9, and 1 mm. (b) The electron energy distribution derived from the images in the left panel.

Figure 6

(a) Wake potential calculated using the measured electron peak current distribution (black curve) and 300-pC bunch charge for various dechirper gap values indicated on the plot. (b) The rms electron bunch energy spread as a function of the dechirper gap. Experimental results are shown with blue squares; calculations are shown with circles.